The present invention relates to dampening systems for offset lithographic printing presses.
The major components of the offset press are the plate cylinder, blanket cylinder, dampening system, and inking system. Secured around the plate cylinder is a planographic printing plate upon which is imposed an image defined by a gradation of oleophilic dots and a hydrophilic background, the intensity of the image being directly proportional to the concentration of such dots.
The function of the dampening system is to thoroughly moisten only the background of the plate with a substantially aqueous solution. Grease-based ink is applied to the plate and adhers only to the oleophilic dots to create an inked image which is transferred to an adjacent blanket cylinder and subsequently to the paper to be printed. The most common dampening devices used in offset lithographic presses are either of the type disclosed in Dahlgren U.S. Pat. No. 4,088,074 issued May 9, 1978, or of the oscillating ductor roller type which has been used in this art for many years.
The dampening system is a critical element of the press. If the quantity and application of the dampening solution is not precise, the solution and ink will incorrectly coat the respective hydrophilic and oleophilic areas, and the finally printed image will be distorted. Conventional ductor roller dampening systems control the amount of solution transferred to the form roller, which contacts the plate cylinder, by adjusting the rate of oscillations of the ductor roller which reciprocates back and forth from a solution reservoir to the train of dampening rollers. The amount of solution which is then actually transferred to the plate is controlled by adjusting the pressure between the form roller and the plate cylinder. Excess solution is nipped between the form roller and the plate cylinder and accumulates on the form roller. This excess solution must eventually be reduced by decreasing the number of oscillations of the ductor roller, or it will accumulate enough to result in over-moistening of the printing plate.
Additionally, in order to absorb a sufficient quantity of dampening solution for subsequent application to the printing plate, the form rollers are covered with a highly absorptive material such as a molleton. The covers, however, ultimately become fouled with ink which collects and finally prevents moistening solution from transferring to the background areas of the printing plate, until such covers are replaced. As a result of these and other disadvantages of prior art dampening systems, sharply defined prints are difficult to obtain without a great deal of control and attention.